1. Field of the Invention
The invention relates to rotary drag-type drill bits, for use in drilling or coring holes in subsurface formations, and of the kind comprising a bit body having an end face and a shank for connection to a drill string, a plurality of blades upstanding from the end face of the bit body and extending outwardly away from the central axis of rotation of the bit, a plurality of cutters mounted on each blade, and a plurality of nozzles in the bit body for delivering drilling fluid to the end face thereof for cooling and cleaning the cutters. Each cutter may include a preform cutting element of the kind comprising a front facing table of superhard material bonded to a less hard substrate. The cutting element may be mounted on a carrier, also of a material which is less hard than the superhard material, which is mounted on the body of the drill bit, for example, is secured within a socket on the bit body. Alternatively, the cutting element may be mounted directly on the bit body, for example the substrate may be of sufficient axial length that it may itself be secured within a socket on the bit body.
2. Description of Related Art
In drag-type drill bits of this kind the bit body may be machined from metal, usually steel, and sockets to receive the carriers or the cutting elements themselves are machined in the bit body. Alternatively, the bit body may be moulded from tungsten carbide matrix material using a powder metallurgy process.
In prior art drag-type drill bits where the cutters are mounted on blades extending outwardly away from the central axis of rotation of the bit, it is usual for each blade, at its outer end, to join a respective kicker which, in use, engages the surrounding wall of the borehole being drilled. The kickers are spaced apart around a peripheral gauge portion of the bit so as to define between the kickers junk slots through which drilling fluid flows from the end face of the bit to the annulus between the drill string and the walls of the borehole. Since it is desirable for the cutters on the blades to define a cutting profile which extends over substantially the whole of the bottom surface of the borehole, it is necessary for at least some of the blades to extend substantially all the way from the central of the end face of the bit outwardly to the gauge of the bit. However, such arrangement inhibits the flow of drilling fluid across the blades in the circumferential direction. Also, if the total number of blades is reduced to improve cutting effectiveness, the stability of the bit may be compromised. The present invention therefore sets out to provide a novel arrangement of blades on a drag-type drill bit whereby these disadvantages of prior art constructions may be reduced or overcome.